This issue of unfocused JTabbedPane having white text on grey background (as opposed to black text on grey background in unfoucsed native app) is long standing issue and I can see from jdk8 to current jdk18 and is not limited to jdk11u on osx10.15.7..

Test used for verification: Used the SwingSet2 Demo.
Product tested : JDK 11.0.13+4 (64 bit)
OS/architecture: MacOS Catalina 10.15.7 / x86_64

STEPS TO REPRODUCE :
1) Run SwingSet2 (MacOSX Look and Feel) and when the Frame is shown, move the focus to any native app and Observe JTabbedPane tab title text .
JTabbedPane tab title text is white text on a light gray background which is barely legible.

It should not be compared to BigSur as the tabbedpane and other components background color design has been inverted by Apple as compared to Catalina and Apple's design is not backward compatible unlike JDK so we cannot expect same design to work for Catalina. That is why we used JRSUIUtils.isMacOSXBigSurOrAbove() check in 8251377 to have different drawing color for TabbedPane in BigSur vis-a-vis Catalina.

I tried changing https://github.com/openjdk/jdk/blob/master/src/java.desktop/macosx/classes/com/apple/laf/AquaLookAndFeel.java#L330
to red color just to test but it does not change, so it seems to be controlled by JavaRuntimeSupport code, similar to JDK-8261243 and that library is handled by Apple.